I ran into a nice surprise on a recent trip.  The Surepower 1315 installed
in my '90 Westy works in two directions.  I'd missed this feature in reading
the specs but re reading the factory spec sheet confirms what I saw.
For those not familiar with the Surepower Battery Separators they are a
relay controlled device that separates the engine/starting battery from the
auxiliary battery but in a more efficient manner than the battery isolators
that use diodes to achive the same effect.  They are also much easier to
install and wire than the diode isolators. With the Surepower Seperators the
starting battery charges first when the engine is running and when the
voltage level reaches a certain point the aux battery is connected to the
charging circuit and is then charged.  Simple enough.  But wait, there's
more.  What I discovered is that with the engine off, if a charging device
is connected to the aux battery, once it is up to snuff then the starting
battery begins to charge also!  As I said, re reading the specs this should
be expected but I'd missed that feature and was presently surprised to see
this happen. (I have a small digital voltmeter permanently  mounted and
attached to both batteries with a switch to select which battery is being
read).
Oh yes, another feature of the 1315 is that a switch can be added (and some
wiring beefed up) and if the starting battery is low when needed the aux
battery can be paralled into the circuit to get you going.  I point this out
because in the middle of my recent trip the starting battery crapped out
completly and had to be replaced.  I haven't added the extra wiring or
switch so I was not able to take advantage of this feature.  That item is
moving to near the top of the 'To Do' list....